1. Field of the Invention
This invention relates to a pulping process to produce pulps used to make paper or paperboard from corn stalks (or other stalks), cereal straws (wheat, barley, rye, oat, etc.), other grasses, and flax or hemp. This is a high yield chemi-mechanical pulping process.
2. Description of the Prior Art
Softwoods, hardwoods, and non-wood plants such as bamboo, bagasse, rice, and wheat straws have been used in the prior art to produce paper pulps used in the world pulp and paper industry. The use of corn stalk to obtain useful fibers for the pulp and paper industry is also known. In the past, the conventional pulping process commonly used with corn stalks gives yields below 45 percent. Similar yields were obtained with straws and grasses.
Notwithstanding that non-wood fibers are typically shorter and more brittle papermaking fibers than softwood fibers (3-5 mm) and, as disclosed in the prior art, that the yield per weight of dry fiber obtained with the current pulping processes for those plants tends to be low, the availability by geographical distribution of those plants is so broad that it makes it possible to consider their use in paper pulps. Typically, corn stalks and corn husks produce fiber pulps from 1 to 2 mm in length, hardwoods produce fibers from 0.8 to 1.5 mm in length. Flax and true hemp produce bast fibers 20 to 25 mm in length, but they require special fiber treatment after cooking (heating).
Once a plant has been selected, such as corn stalk, the plant material must first be cooked or processed through a stage called pulping to remove extraneous materials such as sugars, starches, wax, and most important, lignin. Lignin essentially has to be dissolved because it is a glue-like substance that holds the fibers together. In order to provide for a usable fiber product from the pulping stage, the part of the lignin is dissolved and eventually removed. Then, the fiber is post-treated (screened, bleached, and lightly refined) to make it suitable for papermaking.
Prior to the chemical cooking process, wherein most of the lignin is dissolved and removed, it is important to prepare the specific material, such as corn stalk, to the proper size. Typically, whether it is wood or corn stalk, the material is cut into small chips, preferably between 1/4 inch to 1 inch square, and no longer than 1 inch in any one dimension (in the case of wood only, 1/8 inch thick).
In a conventional corn stalk pulping and bleaching process, the corn stalks are cut into small pieces, typically approximately 1 inch in any one dimension, and screened to sort the chips by size to retain those chips below and over a certain size. Once sorted by size, the chips are washed and then passed to a steaming vessel where air is driven out of the chips by raising the temperature up to 240.degree. F., allowing a cooking liquor to impregnate the chips. The chips pass from the steaming vessel into a high-pressure feeder. Chips and liquor are mixed as the chips are conveyed to the top of a digester. The top section of the digester is pressurized to 160 psi and, as the chip mass passes downward, the cooking liquor (usually soda (NaOH) or neutral sulfite (Na.sub.2 SO.sub.2 +NaOH)) penetrates into the chips. After about 45 minutes, the chips have passed through the impregnation zone and reach a heating zone where the hot liquor (340.degree. F.) is recirculated through the chips for heating. The actual pulping wich is a delignification process occurs at about 335.degree. F., in about 90 minutes, in the next zone coming which is called the cooking zone. The pulp is continuously blown from the digester to a diffusion washer where pulp washing is completed. In a conventional pulping process, the total elapsed time from the time the chips enter the chip bin until they leave the digester as pulp can be about 5 hours.
There are also today some chemi-mechanical pulping processes that are used to convert stalks (bagasse) and straws (rice) into fiber pulps. Those processes use pressurized digesters (either continuous or batch) to cook the pulp and refiners to complete the fiber separation. The yields of those processes may range from 55 to 70 percent, using chemicals such as soda (NaOH), lime soda (CaO.sub.2 NaOH), or neutral sulfite (Na.sub.2 SO.sub.2 +NaOH).
The present invention provides a corn stalk pulping process that can increase the fiber yield of the pulping process up to an average of 75 percent, efficiently and economically. The pulping is done in a high-consistency pulper with the presence of caustic potash (KOH), with or without potassium sulfite (K.sub.2 SO.sub.2) at a relatively low temperature and for a short duration. The Applicant has found that by utilizing this new high yield chemi-mechanical pulping process and preconditioning the corn stalks days before processing, the pulping stage is improved, and the yield of useful fibers from corn stalks (and other cereal straws or grasses) can increase dramatically from 40 up to 75 percent.
Besides, the pulp so produced shows an equal or better quality than commercial pulps on the marked (deinked pulp, kraft pulp from hardwood, BOTMP pulp from hardwood, groundwood, and most agripulps from bamboo and bagasse for example), in particular for its tensile strength, burst strength, and bulkiness (absorbency). Moreover, the type of chemical products used in the pulping process gives residual liquors with high fertilizing potential which are environment friendly.
Finally, no known high yield pulping process exists today for producing pulps from corn stalk (or corn stover, including corn husks) with paper properties comparing with hardwood kraft commercial pulps.